x86: Don't cast away the __user in __get_user_asm_u64()

Don't cast away the __user in __get_user_asm_u64() on x86-32.
Prevents sparse getting upset.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Benjamin LaHaise <bcrl@kvack.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/20170912164000.13745-1-ville.syrjala@linux.intel.com